Hemihydrate or waste anhydrite in composite binders with blast-furnace slag: Hydration products, microstructures and dimensional stability

•We studied composite binders of hemihydrate or anhydrite and blast-furnace slag.•The main hydration products are gypsum, amorphous C–S–H and ettringite.•Gypsum from anhydrite is less porous than the gypsum matrix from the hemihydrate.•Excellent dimensional stability is observed when silica fume and fly ash are used.•C–S–H with Ca/Si composition from 1.08 to 1.78 was intimately mixed with the gypsum.

This paper presents results of an investigation of calcium sulphate-blast furnace slag binders cured for up to 600 days with small additions of fly-ash or silica fume. Hemihydrate and fluorgypsum were used as the calcium sulphate sources. The binders showed good mechanical and hydraulic properties; the water resistance was attributed to the formation of C–S–H with Ca/Si ratio between 1.08 and 1.78 finely intermixed with the gypsum initially formed. Ettringite was formed after the reaction between gypsum and aluminum from the slag and fly ash; however, its formation was hindered in the presence of silica fume, which improved the dimensional stability.